In the field of radio frequency engineering, it is advantageous and usual to evaluate radio-frequency microwave signals not directly but in relation to a reference signal. This relates, for example, to systems for data transmission in which a transmitter, which will be called base station in the text which follows, sends a base signal and this base signal is compared with a reference signal which is generated in a receiver, and processed further in a receiving station. Thus, for example, mixers or demodulators are frequently used by means of which the received signal is down converted into a band of in most cases lower frequency by means of a reference signal. Since, as a rule, the radio-frequency base signal is only used as a carrier onto which a modulation or information of lower frequency is impressed, it is possible, for example, to suppress the carrier by means of this conversion and thus to derive the information contained in the modulation in a simpler manner.
In so-called transponder, transceiver, backscatter or also radar systems, a base signal, which is also called an interrogating signal in this case, is sent by the base station to the transponder or to a reflector and from here is transmitted back to the base station as a response signal, possibly modified, and is there received again. In most cases, the evaluation in the base station then takes place in such a manner that the transmitted base signal itself is used as the reference signal by means of which the response signal is evaluated in order to derive in this way, for example, information added in the transponder or a sensor information item such as, e.g. the delay of the signal and thus the length of the transmission link.
In such systems it is also usual that in the transponder, the received base signal is also processed with a reference signal before a response signal is sent back or, respectively, the reference signal itself, possibly with a characteristic information item added, is sent back to the base station. Such transponders with their own source for sending back the response will be called active transponders or active backscatter devices in the text which follows. By comparison, systems without their own source, that is to say those which only send back the base signal, possibly modified and amplified, are called passive.
In all cases it is advantageous if the reference signal is related as precisely as possible to the base signal or to its carrier with respect to frequency and phase. The more precise this frequency and phase relation, the simpler and/or the more interference-proof is the manner in which the information contained in the base signal or in the response signal can be derived. If the base signal is sent by a base station and received and processed further in the manner described in a spatially distant receiving station, this desired frequency and phase relation is not readily given since both signals, that is to say the base signal generated in the base station and the reference signal generated in the receiving station come from different sources.
For the above reasons, therefore, it is of general interest to couple the reference signal to the base signal in some way. For this purpose, different methods and arrangements are normally used. A simple frequency relation can be implemented by using oscillators with high frequency stability in the transmitter and in the receiver. However, an unknown residual frequency offset will always still remain in this case due to, for example, temperature or aging drift. For this reason, the phases of the two sources cannot have a fixed relation. More elaborate arrangements have means which are suitable for determining the residual frequency offset and/or the residual phase offset. On the basis of the deviation quantities determined, the base signal source or the reference signal source can then be controlled. For this purpose, different frequency and phase control loops are used. Similarly, additional interrogating signals or quantities can be formed from the residual signals, which are utilized for further signal processing. In the field of communications technology, a variety of methods for recovering a carrier are commonly used. The synchronization of oscillators by means of so-called “injection locking” also belongs to the prior art, see, for example, M. Wollitzer, J. Buechler and E. Bibbl, “Supramonic Injection Locking Slot Oscillators”, Electronics Letters, 1993, Vol. 29, No. 22, pages 1958 to 1959. In this case, the oscillator to be controlled is in most cases locked onto a strong stable oscillator. The locking is usually done in CW (Continuous-Wave) mode and subharmonic oscillation modes can also be used for the application. In general, controlling the reference source on the basis of an interrogating signal becomes susceptible to interference and complicated, in particular, if the receiving station is operating not only as a pure receiver but sends back the interrogating signal, possibly provided with an additional information item, as response signal as a transponder, transceiver or active backscatterer. In this case, so-called multiplexing methods must be used for ensuring that the response signal which, as a rule, has a much higher amplitude than the interrogating signal, is not cross-coupled onto the receiving branch and/or onto the control loop. For example, time-division, frequency-division or polarization-division multiplexing methods are normally used. In the case of time-division multiplex, the receiving station first responds to the interrogating signal with a skew. The greater the skew and/or the higher the microwave frequency, the more complicated it is to maintain phase coherence between the source of the base station and that of the transponder. Even extremely small relative frequency deviations of the sources which cannot be avoided due to drift effects, phase noise and control inaccuracies lead to an undefined phase relationship of the sources within a relatively short time in the case of signals of very high frequency. In the case of frequency-division multiplex, the interrogating signal is converted to another frequency in the transponder before it is sent back. This requires dividers, multipliers or additional signal sources and mixers and possibly a number of antennas which are tuned to the respective frequencies. In practice, the principle of frequency multiplication or division also frequently fails because of the radio licensing since, as a rule, the frequencies of the released bands do not have an integral-numbered dividing ratio.
If it is intended to determine the distance or a change in distance between a base station and a transponder, for example in accordance with the principle of the Doppler or frequency-modulation radar, there are even further demands on the phase relation between the interrogating signal transmitted and the response signal sent back. In this case, the phase of the response signal sent back by the transponder must exactly correspond to the phase of the signal received in the transponder, if necessary apart from a constant offset, so that the interrogating signal sent by the base station and the response signal received by it after having been sent back by the transponder have a phase difference which is proportional to the distance between the base station and the transponder but otherwise does not change with time.
Since, in practice, it is only with great difficulty that this phase coherence between two radio-frequency sources can be achieved, passive backscattering transponders which do not have their own signal sources but only reflect back the interrogating signal, possibly amplified, are currently used in most cases. Such systems are described, for example, in Klaus Finkenzeller “RFID-Handbuch” [RFID Manual], second edition, Carl Hanser Verlag, Munich 1999. The disadvantageous factor in such passive backscattering systems is that the transmitted signal must travel along the path from the base station to the transponder as an interrogating signal and back as a response signal and, therefore, the signal/noise ratio of the entire transmission link decreases in proportion to the fourth power of the distance. Because the free-field loss greatly increases with frequency, it is scarcely possible to implement, in particular, passive backscattering transponders of very high frequency in the Gigahertz range with a satisfactory signal/noise ratio. This is unsatisfactory, in particular, because, in principle, because of the great bandwidth available, Gigahertz systems can be very advantageously used both for range finding and for fast data transmission.
In addition, there are systems in which the base signal is not simply reflected and possibly also amplified but in which the response signal is actively constructed on the basis of the base signal, e.g. by means of an active oscillator. For the active design, the relevant parameters are extracted from the base signal and the oscillator signal is generated independently on the basis of the extracted parameters. It represents a reconstruction of the base signal inasmuch as it corresponds to it in the required parameters. Beyond the mere reconstruction, other signal components can also be impressed on the oscillator signal in order to transmit, e.g. additional information.
If a new signal is generated in the transponder in this manner on the basis of a received signal by means of an active oscillator as independent source, the path from the base station to the transponder is in each case only traveled once by the signal from a source. In this case, the signal/noise ratio is only inversely proportional to the power of two of the distance. To this is added that other attenuations and losses on the transmission path act on the signal only once and not twice. The signal/noise ratio is, therefore, particularly in the case of greater distances and/or high frequencies, better by orders of magnitude than in the case of passive backscattering systems in which the signal must travel to and fro on the path from the base station to the transponder.
A more complex transponder system in which the transponder backscatterer operates with its own source is specified in German patent application 19946168.6. This system operates in time-division multiplex and bypasses some of the disadvantages shown by means of a clever choice of modulation and control. However, it is relatively elaborate. It uses the methods which are used in GPS (Global Positioning System). Other systems are mentioned, for example, in U.S. Pat. No. 5,453,748 or in C. Luxey, J.-M. Laheurte “A Retrodirective Transponder with Polarization Duplexing for Dedicated short-range Communications”, IEEE Transactions on Microwaves Theory and Technics, Vol. 47, No. 9, pages 1910 to 1915, or in M. M. Kaleja et al., “Imaging RFID System at 24 Gigahertz for Object Localization, 1999 IEEE MTT-S International Microwave Symposium, Anna Hein, USA, Vol. 4, pages 1497 to 1500.